This work is concerned with the investigation of the mechanisms by which genes affect processes of embryonic differentiation in mammals. Material for these studies is obtained from mutations available in our mutant mouse colony. Among the mutations studied are those which affect the earliest stages of embryonic differentiation into different cell types, the differentiation of elements of the kidney, of muscle cells of the nervous system and of derivatives of the neural crest. The effects of mutant genes are studied on different levels, i.e., those of organs, tissues, cells as well as the ultrastructural and the biochemical levels. Methods employed include those of genetics as well as experimental embryology, histochemistry, tissue culture, electrophoresis, electron microscopy and biochemistry.